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Long before Subaru and Seishirou crossed each other's path at Ikebukuro station, the Sakurazukamori made a bet with him under a cherry tree that they would meet again someday. It happened when Subaru was just nine years old while Lady Sumeragi was taking the young apprentice out for a walk in Ueno Park. Somehow the young Subaru wandered off and lost track of his grandmother. It was the spring season when the cherry tree was in full bloom and shedding soft pink petals all over the park ground. Subaru adored the cherry blossoms and had always been fascinated by their tender beauty. However, Seishirou Sakurazuka, an 18 year old boy also happened to be there. His arms were carrying a dead girl whom he'd just killed. Subaru was shocked to see this happening, but Seishirou soon began to explain just what made the blossoms so beautiful.
According to the Sakurazukamori, cherry blossoms have not always possessed such a beautiful shade of crimson. They used to pure white, but the tree will drink the blood of the corpse buried beneath it and become pale pink blossoms. Young Subaru was horrified by what he was hearing and replied with teary eyes: "Doesn't that hurt the person underneath the tree?" The Sakurazukamori was so taken aback by the boy's innocence that he decided to make a bet with him. 
Since the unwritten law of the Sakurazukamori states that anyone who has witnessed a killing by the clan must also be killed, Seishirou was going to make an exception by sparing his life. Kneeling down with his lips curved into a sinister smile, Seishirou explains that they will meet again when Subaru is older and when they do, he is to spend a year with the boy during which he must decide whether he has feelings for him or not. If Seishirou doesn't feel an ounce of love for Subaru by the time the year ended, he must kill him. If he does, Subaru will live.
Seven years later, Subaru obviously doesn't remember much of the incident. One fateful day, his shikigami flew away from him and he chased it to Ikebukuro station. The pesky bird flew too fast, however, and poor Subaru fell flat onto his face amidst the crowd. At that time, Seishirou was standing on the opposite platform, reading a newspaper and waiting to catch the train home from work - waiting for Subaru just as he promised seven years ago. Subaru, who sustained some minor injury, was taken away to have his wounds bandaged by a kind veterinarian who introduced himself as Seishirou Sakurazuka. Hokuto is a sucker for romantic stories and always teases Subaru about his first meeting with Seishirou. The twins knew that Seishirou was the heir to the Sakurazukamori, another onmyoujitsu clan that controls Japan from the shadows, but they hardly suspect that he was the same man Subaru had met when he was a child. Sometimes, Subaru is amazed by how Seishirou resembled the strange boy when he's struck by odd flashbacks. When they visit Ueno Park together, Seishirou will look at the cherry tree and take off his glasses, telling Subaru that it's a corpse beneath the tree that gives the blossoms their lush color. It seems such a freaky coincidence that Seishirou would say this and scrutinize him with such deep, prowling eyes, but just as Subaru stares at him in disbelief, Seishirou will go back to normal and dismiss it as a joke...